One of the biggest problems caused from vehicles using fossil fuel, such as gasoline and diesel oil, is creation of air pollution. A technology for using a secondary battery, which can be charged and discharged, as a power source for vehicles has attracted considerable attention as one method of solving the above-mentioned problem. As a result, electric vehicles (EV), which are operated using only a battery, and hybrid electric vehicles (HEV), which jointly use a battery and a conventional engine, have been developed. Some of the electric vehicles and the hybrid electric vehicles are now being commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). In recent years, however, the use of a lithium-ion secondary battery has also been attempted.
High output and large capacity are needed for such a secondary battery to be used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). To this end, a plurality of small-sized secondary batteries (unit cells) are connected in series and/or in parallel with each other so as to constitute a battery module, and a plurality of battery modules are connected in parallel and/or in series with each other so as to constitute a middle- or large-sized battery pack.
In such a high-output, large-capacity battery pack, however, a large amount of heat is generated from the unit cells during the charge and discharge of the unit cells. When the heat generated from the unit cells during the charge and discharge of the unit cells is not effectively removed, heat is accumulated in the unit cells with the result that the unit cells are degraded. Furthermore, when some of the unit cells are overheated due to various causes during the accumulation of heat in the unit cells, there is a high possibility that the unit cells catch fire or explode. Consequently, it is necessary to provide a cooling system for cooling a middle- or large-sized battery pack having high output and large capacity.
Generally, the cooling of the middle- or large-sized battery pack is accomplished by the flow of a coolant. For example, there is being used a coolant-flow cooling system constructed in a structure in which a coolant, such as air, forcibly flows between unit cells or battery modules of the battery pack by the operation of a cooling fan. However, this coolant-flow cooling system has several problems.
First, the conventional cooling system has a problem in that the temperature difference between the unit cells is very large. When the battery pack includes a plurality of unit cells, and the unit cells are maintained in optimum operation, the battery pack is also maintained in optimum operation. Consequently, the large temperature difference between the unit cells accelerates the degradation of the unit cells and restrains the optimum operation of the battery pack.
Second, the conventional cooling system increases the size of the battery pack. For example, since the size of a battery pack that can be mounted in electric vehicles (EV) and hybrid electric vehicles (HEV) is restricted, the large-sized battery pack is difficult to be mounted in the electric vehicles (EV) and the hybrid electric vehicles (HEV). FIG. 1 is a typical view illustrating a conventional representative battery pack cooling system.
The battery pack cooling system 10 includes a battery pack 20 comprising a plurality of batteries, a coolant inlet port 30 mounted at the top of the battery pack 20, and a coolant outlet port 40 mounted at the bottom of the battery pack 20. The battery pack 20 comprises a plurality of battery groups 50 electrically connected with each other. Each battery group 50 comprises a plurality of unit cells 60 electrically connected with each other. Between the respective unit cells 60 of each battery group 50 are formed small gaps, through which a coolant flows. Consequently, a coolant introduced through the coolant inlet port 30 flows through the gaps defined between the respective unit cells 60 of each battery group 50 so as to remove heat generated from the respective unit cells 60, and is then discharged through the coolant outlet port 40 mounted at the top of the battery pack 20.
In the above-described structure, the coolant inlet port 30 and the coolant outlet port 40 are mounted at the top and bottom of the battery pack 20, respectively. Consequently, it is required that spaces necessary to mount such coolant guide members be provided at the top and bottom of the battery pack 20. This is a principal factor increasing the total size of the battery pack 20.
In addition, vehicles, such as electric vehicles (EV) and hybrid electric vehicles (HEV), may be frequently operated under tough conditions. The optimum operating condition of each unit cell constituting the battery pack may be changed depending upon various factors. Generally, the optimum operating condition of each unit cell is decided within a specific temperature range. On the other hand, the vehicles are operated at low temperature in the winter season. Consequently, it is required that the battery pack be controlled to be within the above-mentioned optimum operating temperature range. In this case, it is necessary to stop the operation of the cooling system such that the battery pack can be operated within the optimum operating temperature range. Alternatively, it may be necessary to increase the temperature of a coolant (e.g., air) introduced into the cooling system such that the operating temperature of the battery pack can be controlled to be within the optimum operating temperature range. However, when the unit cells of the battery pack have low temperature, battery components may be damaged. Furthermore, the degradation of the battery components may be accelerated due to the abrupt increase of the temperature of the coolant introduced into the cooling system.
Meanwhile, when, in spite of removal of foreign matter by filtering the coolant, some of the foreign matter is introduced into the battery pack, and is brought into contact with the unit cells of the battery pack, another problem may be caused. Generally, the unit cells constituting the battery pack are wrapped with prismatic containers or pouch-shaped laminate sheets. The outer surfaces of the unit cells may be physically or chemically damaged depending upon kinds of foreign matter. Such damage to the unit cells, in which electrochemical reactions occur, causes combustion or explosion of the unit cells.
Consequently, there is high necessity for a technology of fundamentally solving the above-mentioned problems.